1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method and apparatus for transmitting a positioning reference signal in a wireless communication system.
2. Related Art
The next-generation multimedia wireless communication systems which are recently being actively researched are required to process and transmit various pieces of information, such as video and wireless data as well as the initial voice-centered services. The 4th generation wireless communication systems which are now being developed subsequently to the 3rd generation wireless communication systems are aiming at supporting high-speed data service of downlink 1 Gbps (Gigabits per second) and uplink 500 Mbps (Megabits per second). The object of the wireless communication system is to establish reliable communications between a number of users irrespective of their positions and mobility. However, a wireless channel has abnormal characteristics, such as path loss, noise, a fading phenomenon due to multi-path, Inter-Symbol Interference (ISI), and the Doppler Effect resulting from the mobility of a user equipment. A variety of techniques are being developed in order to overcome the abnormal characteristics of the wireless channel and to increase the reliability of wireless communication.
Technology for supporting reliable and high-speed data service includes Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), and so on. An OFDM system is being considered after the 3rd generation system which is able to attenuate the ISI effect with low complexity. The OFDM system converts symbols, received in series, into N (N is a natural number) parallel symbols and transmits them on respective separated N subcarriers. The subcarriers maintain orthogonality in the frequency domain. It is expected that the market for mobile communication will shift from the existing Code Division Multiple Access (CDMA) system to an OFDM-based system. MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology includes spatial multiplexing, transmit diversity, beam-forming and the like. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
In wireless communication systems, it is necessary to estimate an uplink channel or a downlink channel for the purpose of the transmission and reception of data, the acquisition of system synchronization, and the feedback of channel information. In wireless communication system environments, fading is generated because of multi-path time latency. A process of restoring a transmit signal by compensating for the distortion of the signal resulting from a sudden change in the environment due to such fading is referred to as channel estimation. It is also necessary to measure the state of a channel for a cell to which a user equipment belongs or other cells. To estimate a channel or measure the state of a channel, a Reference Signal (RS) which is known to both a transmitter and a receiver can be used.
A subcarrier used to transmit the reference signal is referred to as a reference signal subcarrier, and a subcarrier used to transmit data is referred to as a data subcarrier. In an OFDM system, a method of assigning the reference signal includes a method of assigning the reference signal to all the subcarriers and a method of assigning the reference signal between data subcarriers. The method of assigning the reference signal to all the subcarriers is performed using a signal including only the reference signal, such as a preamble signal, in order to obtain the throughput of channel estimation. If this method is used, the performance of channel estimation can be improved as compared with the method of assigning the reference signal between data subcarriers because the density of reference signals is in general high. However, since the amount of transmitted data is small in the method of assigning the reference signal to all the subcarriers, the method of assigning the reference signal between data subcarriers is used in order to increase the amount of transmitted data. If the method of assigning the reference signal between data subcarriers is used, the performance of channel estimation can be deteriorated because the density of reference signals is low. Accordingly, the reference signals should be properly arranged in order to minimize such deterioration.
A receiver can estimate a channel by separating information about a reference signal from a received signal because it knows the information about a reference signal and can accurately estimate data, transmitted by a transmit stage, by compensating for an estimated channel value. Assuming that the reference signal transmitted by the transmitter is p, channel information experienced by the reference signal during transmission is h, thermal noise occurring in the receiver is n, and the signal received by the receiver is y, it can result in y=h·p+n. Here, since the receiver already knows the reference signal p, it can estimate a channel information value ĥ using Equation 1 in the case in which a Least Square (LS) method is used.ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Equation 1]
The accuracy of the channel estimation value ĥ estimated using the reference signal p is determined by the value {circumflex over (n)}. To accurately estimate the value h, the value {circumflex over (n)} must converge on 0. To this end, the influence of the value {circumflex over (n)} has to be minimized by estimating a channel using a large number of reference signals. A variety of algorithms for a better channel estimation performance may exist.
Meanwhile, UE positioning for estimating a location of a UE has been recently used for diverse purposes in real life, and thus, a precise UE positioning method is required. A UE positioning technique may be divided into the following four methods.
1) Cell ID-based method: A cell ID-based method uses a cell coverage. A location of a UE can be estimated from information regarding a serving cell which serves the corresponding UE. The information regarding the serving cell may be obtained through paging, locating area updating, cell updating, URA updating, routing area updating, or the like. Positioning information based on a cell coverage may be indicated through a cell identity of the cell in use, a service area identity, or geographical coordinates in relation to the serving cell. The positioning information may include QoS (Quality of Service) estimation information, and may include information regarding a positioning method used to estimate a position if possible. When geographical coordinates are used as positioning information, an estimated location of a UE may be any one of a certain fixed location within the serving cell, a geographic central point of the coverage of the serving cell, or a different fixed location within the cell coverage. Also, the geographical location may be obtained by combining information regarding the cell-specific fixed geographical location and different information. The different information may be information such as an RTT (Round Trip Time) of a signal in an FDD (Frequency Division Duplex) mode, a reception timing deviation in a TDD mode, or the like.
2) OTDOA-IPDL (Observed Time Difference of Arrival-Idle Periods in Downlink) method: FIG. 1 shows the concept of a location estimation of a UE by an OTDOA-IPDL method. A location of a UE is estimated by using the difference in timing between signals transmitted from base stations (BSs). When the UE is located to be very close to the serving cell, a hearability problem in which the UE cannot properly receive a signal transmitted by a neighbor cell due to the intensive transmission power of the serving cell may arise. This is because an ADC level is determined based on the serving cell and signals transmitted from neighbor cells are received at a level lower than the ADC level, making it impossible to discriminate the signals. Thus, in order to solve this problem, IPDL may be applied to downlink of the serving cell. IPDL can be set in a network. In the OTDOA-IPDL method, when an idle period is not used, the OTDOA-IPDL method is a simple OTDOA method.
3) Network-supported GNSS (Global Navigation Satellite System) method: In this method, a terminal including a receiver capable of receiving a GNSS signal is used. In order to estimate a location of the terminal, various types of GNSS signals may be independently used or combined to be used.
4) U-TDOA method: This method is given on the basis that a network measures a TOA (Time of Arrival) of a signal which is transmitted from a UE and received by four or more BSs. In this method, in order to accurately measure a TOA of data, a BS which is geographically close to the UE is required. Since geographical coordinates of a measurement unit are already known, a location of the UE can be estimated by hyperbolic trilateration.
In order to estimate a location of a UE, a reference signal may be used. The reference signal may include a synchronization signal. The UE may receive reference signals transmitted from a plurality of cells, and use the difference in a delay time of each signal. The UE may report the difference in the corresponding delay time to the BS to allow the BS to calculate a location of the UE, or the UE itself may calculate its location. With reference to Paragraph LTE TS36.355 V9.0.0 (2009-12) 4.1.1, measurement values such as the difference in the delay time of reference signals (RSTD; Reference Signal Time Difference), transmitted from each cell, measured by the UE can be controlled by E-SMLC (Enhanced Serving Mobile Location Centre) through LPP (LTE Positioning Protocol). The LPP may be defined in a point-to-point manner between a location server such as E-SMLC, or the like, and a target device such as a UE, or the like, in order to a location of the target device by using a location relationship measurement value obtained from one or more reference signals.
The pattern of reference signals transmitted from a plurality of cells to a UE is required to be designed in consideration of a power difference, a delay difference, or the like. A method for effectively designing the structure of a reference signal is required.